KUALA LUMPUR, March 18 (Reuters) - The co-pilot of a missing Malaysian jetliner spoke the last words heard from the cockpit, the airline's chief executive said, as investigators consider suicide by the captain or first officer as one possible explanation for the disappearance.

No trace of Malaysia Airlines Flight MH370 has been found since it vanished on March 8 with 239 people aboard. Investigators are increasingly convinced it was diverted perhaps thousands of miles off course by someone with deep knowledge of the Boeing 777-200ER and commercial navigation.

A search of unprecedented scale involving 26 countries is under way, covering an area stretching from the shores of the Caspian Sea in the north to deep in the southern Indian Ocean.

Airline chief executive Ahmad Jauhari Yahya also told a news conference on Monday that it was unclear exactly when one of the plane's automatic tracking systems had been disabled, appearing to contradict comments by government ministers at the weekend.

Suspicions of hijacking or sabotage had hardened further when officials said on Sunday that the last radio message from the plane - an informal "all right, good night" - was spoken after the tracking system, known as "ACARS", was shut down.

"Initial investigations indicate it was the co-pilot who basically spoke the last time it was recorded on tape," Ahmad Jauhari said on Monday.

That was a sign-off to air traffic controllers at 1.19 a.m., as the Beijing-bound plane left Malaysian airspace.

The last transmission from the ACARS system - a maintenance computer that relays data on the plane's status - was received at 1.07 a.m. as the plane crossed Malaysia's northeast coast.

"We don't know when the ACARS was switched off after that," Ahmad Jauhari said. "It was supposed to transmit 30 minutes from there, but that transmission did not come through."

FOCUS ON CREW

The plane vanished from civilian air traffic control screens off Malaysia's east coast less than an hour after taking off from Kuala Lumpur. Malaysian authorities believe that someone on board shut off its communications systems.

Malaysian police are trawling through the backgrounds of the pilots, flight crew and ground staff for any clues to a possible motive in what is now being treated as a criminal investigation.

Asked if suicide by the pilot or co-pilot was a line of inquiry, Malaysian Acting Transport Minister Hishammuddin Hussein said: "We are looking at it." But it was only one of the possibilities under investigation, he said.

Intensive efforts by various governments to investigate the backgrounds of everyone on the airplane had not, as of Monday, turned up any information linking anyone to militant groups or anyone with a known political or criminal motive to crash or hijack the aircraft, U.S. and European security sources said.

One source familiar with U.S. inquiries into the disappearance said the pilots were being studied because of the technical knowledge needed to disable the ACARS system.

Many experts and officials say that, while the jet's transponder can be switched off by flicking a switch in the cockpit, turning off ACARS may have required someone to open a trap door outside the cockpit, climb down into the plane's belly and pull a fuse or circuit breaker.

Whoever did so had to have sophisticated knowledge of the systems on a 777, according to pilots and two current and former U.S. officials close to the investigation.

Malaysian police searched the homes of the captain, Zaharie Ahmad Shah, 53, and first officer, Fariq Abdul Hamid, 27, in middle-class suburbs of Kuala Lumpur close to the international airport on Saturday.

Among the items taken for examination was a flight simulator Zaharie had built in his home but a senior police official familiar with the investigation said there was nothing unusual in the flight simulator programmes. A second senior police official with knowledge of the investigation said they had found no evidence of a link between the pilot and any militant group.

Some U.S. officials have expressed frustration at Malaysia's handling of the investigation. The Malaysian government still had not invited the FBI to send a team to Kuala Lumpur by Monday, two U.S. security officials said.

VAST SEARCH CORRIDORS

Police and the multi-national investigation team may never know for sure what happened in the cockpit unless they find the plane, and that in itself is a daunting challenge.

Satellite data suggests it could be anywhere in either of two vast corridors that arc through much of Asia: one stretching north from Laos to the Caspian, the other south from west of the Indonesian island of Sumatra into the southern Indian Ocean.

Aviation officials in Pakistan, India, and Kazakhstan and Kyrgyzstan in Central Asia - as well as Taliban militants in Pakistan and Afghanistan - said they knew nothing about the whereabouts of the plane.

China, which has been vocal in its impatience with Malaysian efforts to find the plane, called on its smaller neighbour to immediately expand and clarify the scope of the search. About two-thirds of those aboard MH370 were Chinese.

Australia has offered more resources in addition to the two P-3C Orion aircraft it has already committed.

Malaysian Transport Minister Hishammuddin said diplomatic notes had been sent to all countries along the northern and southern search corridors, requesting radar and satellite information as well as land, sea and air search operations.

The Malaysian navy and air force were also searching the southern corridor, he said, and U.S. P-8A Poseidon surveillance aircraft were being sent to Perth, in Western Australia, to help scour the ocean.

At the same time, the U.S. Navy said the destroyer USS Kidd was ending its search operations in the Andaman Sea.

NORTH OR SOUTH?

Electronic signals between the plane and satellites continued to be exchanged for nearly six hours after MH370 flew out of range of Malaysian military radar off the northwest coast, following a commercial aviation route across the Andaman Sea towards India.

The plane had enough fuel to fly for about 30 minutes after that last satellite communication, Ahmad Jauhari said.

A source familiar with official U.S. assessments of satellite data being used to try to find the plane said it most likely turned south after the last Malaysian military radar sighting and may have run out of fuel over the Indian Ocean.

The Malaysian government-controlled New Straits Times on Monday quoted sources close to the investigation as saying data collected was pointing instead towards the northern corridor.

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